Heretofore, a high zirconia fused cast refractory comprising at least 80 mass % of ZrO2 as a chemical component has been used as a refractory for a glass melting furnace. A high zirconia fused cast refractory, which has high corrosion resistance against molten glass and low contamination properties, has been frequently used for a portion of contact with molten glass of a glass melting furnace which is required to have high quality, such as substrate glass for a flat panel display.
The microstructure of a high zirconia fused cast refractory is composed of a slight quantity of pores, a large amount of zirconia (ZrO2) crystal grains and a small amount of matrix glass with which the space between the grains is filled. This matrix glass contains SiO2 as the main component and contains other oxides, such as Al2O3, Na2O, B2O3 and P2O5.
A high zirconia fused cast refractory is exposed to temperature changes in cooling step at the time of its production, and during the heating in a glass melting furnace and during the cooling at the time of suspension of operation, by the process in operation and by corrosion of the refractory itself. By such temperature changes, a thermal stress, and a transformation stress caused by reversible transformation of zirconia crystals accompanied by a significant volume change in a temperature range in the vicinity of 1,000° C., occur in the interior of the refractory. When matrix glass having appropriate thermomechanical properties in an appropriate amount is contained in the refractory, the refractory is flexible against the above stresses, and the stresses are relaxed, whereby no cracks will form on the refractory. In this specification, the fused cast refractory will sometimes be referred to simply as a refractory.
Whereas, if the thermomechanical properties of the matrix glass are inappropriate or if the amount of the matrix glass is insufficient, cracks may form at the time of production of the high zirconia fused cast refractory or during the heating when the refractory is applied to a glass melting furnace. If the refractory has cracks when applied to a portion of contact with molten glass, this portion will be severely corroded by molten glass, whereby the durability of the refractory will significantly be decreased.
In the interior of a high zirconia fused cast refractory, zircon crystals (ZrO2.SiO2) may be formed in some cases. The zircon crystals in the interior of the refractory are formed by reaction of ZrO2 and SiO2 in the matrix glass, and accordingly formation of the zircon crystals leads to a decrease in the matrix glass in the refractory. Such a refractory in which zircon crystals are formed and the amount of the matrix glass which relaxes the thermal stress and the transformation stress is reduced becomes fragile and is likely to have cracks even by a slight temperature change.
Further, even in a high zirconia fused cast refractory in which zircon crystals are hardly formed by the refractory itself, zircon crystals may be formed by a reaction with molten glass in some cases. This is because either one or both of elution of chemical components which suppress formation of zircon crystals contained in the refractory into molten glass, and invasion of chemical components which accelerate formation of zircon crystals into the refractory from molten glass, occurs. The tendency of the zircon crystals to be formed by the reaction with molten glass is remarkable when the refractory is in contact with low alkali glass such as liquid crystal substrate glass or with alkali-free glass.
Accordingly, in a case where a high zirconia fused cast refractory in which zircon crystals are likely to be formed by the thermal history by the refractory itself, or a high zirconia fused cast refractory in which zircon crystals are hardly formed by the refractory itself but zircon crystals are likely to be formed by a reaction with molten glass, is used as a refractory for a glass melting furnace, even when no cracks form at the time of production and even when no cracks form during the heating, zircon crystals may be formed in the interior of the refractory in operation, whereby cracks are likely to form by the temperature changes in operation, and the durability of the refractory is significantly decreased in some cases.
In general, the durability of the refractory is a factor which determines the life of a glass melting furnace. Accordingly, formation of cracks in the refractory shortens the life of a glass melting furnace, which is one cause to increase the cost for glass production.
Further, in a high zirconia fused cast refractory in which no zircon crystals are formed in a state where the glass meting furnace is in operation, no cracks will form, or even if cracks will form, they are few and small as compared with a refractory in which zircon crystals are formed, and formation of new cracks or propagation of existing cracks during the cooling when the operation of the glass melting furnace is suspended for e.g. adjustment of production tends to be little, and accordingly such a refractory is relatively easily reused.
On the other hand, in a high zirconia fused cast refractory in which zircon crystals are formed, formation of new cracks and propagation of existing cracks are remarkable during the cooling, and further, formation of cracks and propagation occur during the heating again, and thus reusing such a refractory is difficult. Even if it is reused, no high durability will be obtained, and the furnace life will be short. That is, a high zirconia fused cast refractory in which zircon crystals are likely to be formed by itself or by a reaction with molten glass, even if it has a remaining life in a state where the glass melting furnace is in operation, is unsuitable for reuse after suspension of operation.
A means to suppress formation of cracks in a high zirconia fused cast refractory at the time of production, during the heating and in operation has been studied.
Patent Document 1 proposes a high zirconia fused cast refractory which has a chemical composition comprising from 85 to 97 mass % of ZrO2, from 2 to 10 mass % of SiO2, at most 3 mass % of Al2O3 and from 0.1 to 3 mass % of P2O5, and containing substantially no rare-earth oxide, whereby cracks to be formed at the time of production are suppressed. However, this refractory contains P2O5 which accelerates formation of zircon crystals, and has a drawback such that zircon crystals are likely to be formed even by the refractory itself.
Patent Document 2 proposes a high zirconia fused cast refractory which has a chemical composition comprising from 90 to 98 mass % of ZrO2 and at most 1 mass % of Al2O3, containing no Li2O, Na2O, CuO, CaO and MgO, and containing from 0.5 to 1.5 mass % of B2O3, or containing from 0.5 to 1.5% of B2O3 and containing at most 1.5% of one member selected from K2O, SrO, BaO, Rb2O and Cs2O or a total content of two or more of them of at most 1.5%, whereby cracking at the time of production is suppressed, a component having a large cation radius is used and the electrical resistivity is high. However, the refractory has a high content of B2O3 which accelerates formation of zircon crystals, and has a drawback such that zircon crystals are likely to be formed even by the refractory itself.
Patent Document 3 proposes a refractory which has a chemical composition comprising from 90 to 95 mass % of ZrO2, from 3.5 to 7 mass % of SiO2, from 1.2 to 3 mass % of Al2O3 and from 0.1 to 0.35 mass % in total of Na2O and/or K2O, and containing substantially no P2O5, B2O3 and CuO, whereby the thermal cycle resistance is improved and formation of zircon crystals is suppressed. However, even the refractory of this invention has an insufficient effect to suppress formation of zircon crystals under conditions of contact with molten glass. Further, at the time of production of the refractory particularly at the time of production of a large-sized refractory with a mass of an ingot of at least 300 kg, cracks are likely to form.
Patent Document 4 proposes a refractory which has a chemical composition comprising from 89 to 96 mass % of ZrO2, from 3.5 to 7 mass % of SiO2, from 0.2 to 1.5 mass % of Al2O3, from 0.05 to 1.0 mass % of Na2O+K2O, less than 1.2 mass % of B2O3, less than 0.5 mass % of P2O5, higher than 0.01 mass % and less than 1.7 mass % of B2O3+P2O5, less than 0.3 mass % of CuO, at most 0.3 mass % of Fe2O3+TiO2, from 0.01 to 0.5 mass % of BaO, and at most 0.3 mass % of SnO2. Patent Document 4 discloses that cracking at the time of production of the refractory and cracking by the thermal cycle will not occur, and further, addition of Na2O, K2O and BaO cause disadvantageous properties of P2O5 and B2O3 which accelerate formation of zircon crystals, to disappear. However, even the refractory of this invention still has an insufficient effect to suppress formation of zircon crystals under conditions of contact with molten glass. The reasons are such that B2O3 and P2O5 having an effect to accelerate formation of zircon crystals are contained in a relatively high content in Examples of this invention, and that the K2O content is not sufficient to the relatively high contents of B2O3 and P2O5.
Patent Document 5 proposes a refractory which has a chemical composition comprising from 87 to 94 mass % of ZrO2, from 3.0 to 8.0 mass % of SiO2, from 1.2 to 3.0 mass % of Al2O3, higher than 0.35 mass % and at most 1.0 mass % of Na2O and higher than 0.02 mass % and less than 0.05 mass % of B2O3, containing substantially no P2O5 and CuO, and having a mass ratio of Al2O3 to Na2O of from 2.5 to 5.0, whereby formation of zircon crystals by the refractory itself is suppressed. However, in this refractory based on this invention, formation of zircon crystals is suppressed by optimizing the content ratio of Na2O and Al2O3, and accordingly under conditions of contact with molten glass containing Na2O only in a low content, elution of Na2O occurs in priority. The refractory has a drawback such that by such elution, the proportion of Na2O and Al2O3 will soon deviate from the initial value in an unused state, the composition of the refractory departs in a short time from a composition advantageous for suppression of formation of zircon crystals, and the effect to suppress formation of zircon crystals obtainable by the refractory itself is soon lost.